1. Field of the Invention
The present invention relates to motor vehicle electrical systems and more particularly to a programmable system protecting such electrical systems from overcurrent conditions.
2. Description of the Problem
The number of electrical circuits in automotive vehicles has increased over the years. In today""s motor vehicles there are numerous electrical devices which are used for various purposes such as illumination, control, power, and instrumentation. While the advent of electronics has given rise to major changes in automotive electrical systems, conventional circuit protection devices, e.g. fuses and circuit breakers, continue to be used, and in increasing numbers as the number of circuits in the electrical systems increases. The common technique for providing protection against shorts, overloads, and other types of electrical problems or conditions is to include a circuit breaker or fuse connected in series, with the wiring circuit to be protected.
With increasing numbers of circuits, and the correlative need for an increased number of protective devices, today""s typical automotive vehicle or truck requires a panel devoted essentially exclusively to the mounting of most of these protective devices in a single location. The panel, or fuse block as it is sometimes called, comprises multiple compartments for the individual protective devices. Associated with these compartments are receptacles to provide for the replaceable mounting of the protective devices in the associated circuits. Accordingly, the panel comprises a large number of individual parts in assembled relationship, and it occupies a certain amount of space in an area of the vehicle where space is typically at a premium. A large number of wires attach to the panel to carry current to and from the various protective devices, and in order to serve the grouping of the protective devices in the panel, complexities are introduced into the associated wiring harnesses and cost is added to the vehicle. In addition, the variability of commercial vehicles may result in different valued fuses or circuit breakers being installed at the same physical location on different vehicles of the same model truck, resulting in assembly errors.
There are several ways to protect an electrical device without a circuit breaker or fuse, but most of the ways add several parts to the circuit and typically degrade the performance of the electrical circuit, such as by added voltage drop, higher power dissipation, etc. These protection methods are not known to enjoy any significant commercial use because of disadvantages such as those just mentioned. Providing a substitute device for a fuse can pose other complications. Devices for interrupting a circuit based on detection of a simple overcurrent condition do not mimic fuse behavior, which is characterized by opening after passage of an overcurrent of a sufficient time duration to cause the fusible element to melt. Fuses thus tolerate transient, non fault related, overcurrent conditions, sometimes greatly exceeding the rated tolerance of the fuse, such as occur when a lamp is turned on. Fuses also tolerate other types of brief overcurrent excursions such as peaks occurring in alternating current circuits, where the root mean square value for the current remains below the direct current rating for the fuse. It is often desirable to use fuses in circuits for just this feature.
U.S. Pat. No. 4,799,126 to Kruse, et al., which is assigned to the assignee of the present invention, recognized that the fuse and circuit breaker panel concept of protection could be eliminated, thereby reducing the large number of individual circuit devices (i.e., fuses and circuit breakers) required to provide the protective function, and at the same time, freeing space because there is no longer a need for a separate panel. The circuit breaker function is provided by using a particular type of power MOSFETs, which also serve for circuit switching. MOSFETs comprise an internal, controlled conduction path the conductivity of which is controlled by an external control input. The type of MOSFET used by Kruse comes with built in protection, contained in another internal portion which monitors current flow through the main controlled conduction path and serves to internally interrupt the flow through the path in response to incipiency of current or temperature exceeding the rating of the main controlled conduction path. When one of these MOSFETs is incorporated into a circuit, it is selected on the basis of a close match in the amount of current to be allowed to be drawn by a circuit and the tolerances of the MOSFET. This final aspect of Kruse""s teaching necessitated manufacturing vehicles using MOSFETs of a number of different capacities.
Kato et al., U.S. Pat. No. 5,856,711 provides a circuit interrupt device capable of being set for different current-time characteristics without physical modification of the device itself. In addition, Kato appears to provide a device which mimics the time delay in breaking inherent to fuses operating under overcurrent conditions. Kato teaches switches (relays) having a control input; current detection functionality; and a load drive line for connection to loads to which electric power is supplied from a battery. A device controller includes data processing capacity and memory, on which is stored the desired current/breaking time characteristics data. The device controller opens a switch by supplying a control input signal to the switch when the breaking time in the memory has elapsed. This is effected by starting a timer immediately after detection of an overcurrent condition and running it against a time out threshold stored in memory for the value of the current. Memory is programmable for the desired time-current values. The ""711 patent does not appear to vary the breaking time period for changes in current once a timer has been started. Thus, so long as an overcurrent condition continues to exist, the timer continues to run against the initial time out period matched with the initially detected overcurrent condition. The timer stops only if current falls below a minimum threshold level. This aspect of Kato""s control algorithm presents difficulties in applying the system to circuits other than those designed for use with clean direct current loads. Kato et al. do not address these problems.
Power MOSFETs are popular switching devices in contemporary vehicle electronics. Among other applications, power MOSFETS can be used to implement pulse width modulation (PWM) switching, which allows precise control over vehicle features such as varying the illumination level of running lights and changing the operating speed of electric motors to change the sweep speed of windshield wipers. PWM is, in effect, an alternating current signal with a direct current offset, or unipolarity A.C. In PWM switching systems, peak values in current drawn may vary, for example changing with the changing load associated with windshield motor operation under conditions windshield icing. Circuit protection devices used with such systems, in order to be effective, must operate accurately in such a quasi or unipolarity alternating current (A.C.) environment. Peak pulse current values may safely exceed the current rating for the circuit without being symptomatic of a dangerous condition or indicative of a short, so long as the root mean square (RMS) value of the current remains below the maximum current rating. Conversely, current drawn may be excessive, but a system such as proposed by Kato et al. would miss detection of it because the duty cycle is to short for the timer to expire.
The Kruse et al. and Kato et al. patents do not address environments where the circuit current has A.C. components, but instead appear limited to D.C. applications. Kato et al. apply data processing capacity to the determination of when to trip a relay in response to excessive current being drawn by a circuit. Though the algorithm employed by Kato et al. appears tolerant of transient overcurrent situations, it does explicitly deal with quasi A.C. conditions.
In addition, MOSFET devices require protection. During high levels of overload, any field effect transistor (FET) will be rapidly heated and cooled as temperature protection mechanisms of the FET limit the power dissipated in the FET. Over time, such heating and cooling of the FET reduce the useful life of the device, an effect known as the Coffin-Manson acceleration.
It would be advantageous in vehicle manufacture to dispense with fuses for circuit protection and implement circuit overcurrent protection directly in the switches used to control the circuits. It would be still more advantageous if the switches were standardized and if implementation of their response characteristics could be introduced to the vehicle by programming. Such a feature would simplify manufacture and repair. It would be still more advantageous if the devices could be programmed to handle a wide variety of different operating conditions, including unipolarity A.C. operation.
According to the invention there is provided a motor vehicle comprising electrical power distribution circuits. Switching elements are incorporated in the electrical circuits for controlling the energization thereof. Current metering elements associated with each switching element indicate the current drawn by the respective electrical circuits. A microcontroller is provided which provides an activation signal for the switching elements, often in accord with a pulse width modulated duty cycle. The microcontroller implements a circuit protective algorithm which takes as inputs the indication of current drawn by a particular electrical circuit and the duty cycle. An equivalent D.C. current is developed for determining a heat index for a hypothetical fuse suitable for protecting the circuit. When the accumulated heat index exceeds the heat index rating for the hypothetical fuse the circuit is opened.
Additional effects, features and advantages will be apparent in the written description that follows.